Body temperature may fall as a result of heat loss by radiation, evaporation, conduction, and convection.2 Radiation causes 55-65% of the body's heat loss. Evaporation occurs via the skin and airway and accounts for 30% of the heat loss. Normally, in a dry environment, only 15% of the body's heat loss results from conduction. However, the thermal conductivity of water is approximately 30 times that of air, so the body loses heat rapidly when immersed in water or covered in wet clothing, leading to a rapid decline in body temperature. Convection accounts for a minor amount of heat loss, but it becomes more significant in a windy environment. The amount of heat dissipated by any of these mechanisms is proportional to the temperature difference between the body and environment.
Opposing the loss of body heat are the mechanisms of heat conservation and gain. In general, a thermostat in the preoptic region of the hypothalamus controls these mechanisms. This human thermostat is set to a precise reference temperature, usually very close to 37°C (98.6°F). It responds to thermoregulatory mechanisms, the temperature of blood, and temperature receptors deep within the body and in the skin.
When the preoptic area of the hypothalamus is stimulated, various heat conservation and production mechanisms become activated. When the sympathetic nerves are excited, they cause the blood vessels in the skin to markedly constrict. The flow of warm blood from the core of the skin is depressed, thereby reducing the transfer of heat to the body surface. This reduction of blood flow in the skin is the prime physiologic regulator of heat loss from the body. The temperature of the skin decreases to approach the temperature of surrounding air, which lowers the temperature gradient and further decreases heat loss.
Stimulation of the sympathetic nerves also causes secretion of epinephrine and norepinephrine by the adrenal medullae. These hormones increase the metabolic rate of all cells, thereby enhancing heat production. Impulses from the preoptic hypothalamus also activate the primary motor center for shivering, which, in turn, increases the tone of muscles. The resulting enhancement of muscle metabolism can increase heat production by as much as 500%.
Causes
Hypothermia or systemic cold injury is a clinical condition in which the core body temperature has decreased to 35°C (95°F) or less. The causes of hypothermia are either primary or secondary. Primary, or accidental, hypothermia occurs in healthy individuals inadequately clothed and exposed to severe cooling.3 Accidental hypothermia can be divided into immersion and nonimmersion cold exposure. The high thermal conductivity of water leads to the rapid development of immersion hypothermia. Although the rate of heat loss is determined by water temperature, immersion in any water less than 16°C (60.8°F) may lead to hypothermia within minutes. (Click here to complete a Medscape CME activity on mild hypothermia therapy.)
When individuals are buried in the snow of an avalanche, they must be extricated from the scene of the avalanche accident as soon as possible.4 In fact, rapid extrication is the most important determinant of positive outcome in snow avalanche victims. To facilitate the rapid localization of avalanche victims, avalanche transceivers are widely used during off-piste and back country activities.
Hohlrieder et al conducted a retrospective study to analyze the influence of transceivers on the mortality of avalanche victims.4 In the 194 accidents in Austria between 1994-2003, 278 victims were totally buried. Avalanche transceivers were used by 156 victims (56%), and transceiver use was associated with a significant reduction in the median burial time, which decreased from 102 minutes to 20 minutes (P < .001). Transceiver use was also associated with a significant reduction in mortality, which decreased from 68.0% to 53.8%. This reduction reflects a decrease in mortality during back country activities that involved ski tourists in free alpine areas. Transceivers did not significantly reduce mortality when they were used in off-piste activities beside or near organized ski slopes.
Even if a person is using a transceiver, mortality is significantly higher if burial depth exceeds 1.5 m. Despite a significant reduction in mortality, mortality still exceeds 50% even with transceivers. Consequently, even with the use of emergency equipment and life transceivers, avoiding avalanches is critically important. The authors conclude that the fairly modest influence of the use of transceivers on survival probability may also be due to the high efficiency of the mountain rescue service in the Austrian Alps.
In secondary hypothermia, another illness predisposes the individual to accidental hypothermia. The mechanism of secondary hypothermia appears to be an acute failure of thermoregulation; shivering does not usually occur in these patients. In many reports, alcohol seems to be a predominant cause of cutaneous vasodilation, loss of shivering, hypothalamic dysfunction, and lack of concern regarding the environment.5 Other factors that predispose an individual to acute hypothermia include the following:
- Endocrine diseases (eg, hypothyroidism, pituitary insufficiency, Addison disease, diabetes mellitus6)
- Cardiovascular diseases (eg, myocardial infarction,7 congestive heart failure, vascular insufficiency)
- Neurologic diseases (eg, cerebrovascular accident, head injury, tumor, spinal cord injury, Alzheimer disease8)
- Drugs (eg, phenothiazines, barbiturates, antidepressants)
- Pancreatitis
- Cirrhosis
- Hypoglycemia
For more information on some of the conditions that predispose individuals to acute hypothermia, visit the following Medscape Resource Centers: Hypothyroidism, Diabetic Microvascular Complications, Heart Failure, and Trauma.
Clinical presentation
Hypothermia affects multiple organs.9 Initially, the metabolic rate increases, with tachycardia, tachypnea, increased muscle tone, and peripheral vascular resistance to generate maximal shivering. With continued hypothermia, the metabolism progressively declines, with bradycardia and hypoventilation and subsequent carbon dioxide retention. The heart rate drops to half its normal rate at 28°C (82.4°F), and ventricular contractility decreases. The risk of ventricular fibrillation increases at temperatures below 28°C (82.4°F). Cerebral metabolism is decreased 6-7% per 1°C drop in temperature, which results in a declining level of consciousness. Autoregulation of cerebral blood flow is impaired at temperatures below 25°C (77°F). The shivering mechanism of thermoregulation stops at 31°C (87.8°F).
The symptoms of hypothermia vary depending on the severity of the cold injury. In mild hypothermia, clinical symptoms are often vague and include dizziness, fatigue, joint stiffness, nausea, and pruritus. The skin is pale and cool as a result of peripheral vasoconstriction. The patient may exhibit lethargy, flat affect, impaired judgment, and mild confusion progressing to motor incoordination, ataxia, and slurred speech.
In severe hypothermia, mental status is further impaired, leading to hallucinations, stupor, and even coma. Atrial and ventricular arrhythmias are common with moderate hypothermia. The Osborn (J) point, an upward deflection at the junction of the QRS complex and the ST segment, can usually be seen on the ECG. The patient may appear clinically dead, with nonpalpable peripheral pulses, fixed and dilated pupils, loss of ocular reflexes, and stiff extensor posturing. Cardiac standstill usually occurs at 20°C (68°F), but one report described a survivor whose temperature was 15°C (60.8°F).
Diagnosis
The diagnosis of hypothermia is easy if the patient is a mountaineer who is stranded in cold weather.10, 11 However, it may be more difficult in an elderly patient who has been exposed to a cold external environment. In either case, the rectal temperature should be checked with a low-reading thermometer. The diagnosis of accidental hypothermia has proved elusive, largely because clinical thermometers do not record temperatures below 35°C (95°F). The only inexpensive low-reading thermometer is the Zeal (Zeal Group Ltd; London). Electronic thermometers with digital readouts and remote electric probes are made by several companies. Rectal temperature measurements are influenced by lower body temperature and probe placement. An inaccurate reading may result if the rectal probe was inserted in cold feces or to a depth of less than 15 cm.
Other methods of determining core body temperature include infrared tympanic thermometers, esophageal probes in intubated patients, and bladder thermistors embedded in a urinary catheter. The tympanic probe accurately measures hypothalamic temperature and most rapidly changes to reflect variations in core body temperature. On the basis of temperature measurements, the arbitrary classification of the level of hypothermia is mild (<34°c>
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